This invention pertains to fusion machines and more particularly to improvements in tokamaks.
Tokamaks are machines originally developed by the Russians starting in 1958. After L. A. Artsimovich came to the United States in 1969 and delivered a series of lectures the first American tokamak was built in Princeton in 1970. The goal of these machines is to attain nuclear fusion.
It is known (the Lawson criterion) that such fusion can be attained with a net power release when a plasma which is a fifty percent mixture of deuterium and a fifty percent mixture of tritium with a number density (n) is maintained for a time (.tau.) so that the combined product is (5.times.10.sup.14 atoms/cm.sup.3 .times.200 msec=10.sup.14 atoms-sec/cm.sup.3) and held for this time at a temperature of 100.times.10.sup.6 degrees Kelvin. If these two conditions of the number density-time product and the temperature are achieved the amount of electrical energy produced by the fusion of the hydrogen isotopes deuterium and tritium will equal the electrical energy required to produce the plasma. In addition, it is required that heat energy of the plasma be recovered and transformed into electricity at 33% efficiency while the plasma itself is electrically heated at 100% efficiency. It is believed that if the values of the number density-time product and the temperature were increased by only a factor of two or three there would be a large ratio of output power over the input power.
The basic design of a tokamak as given by L. A. Artsimovich comprises a stainless steel chamber filled with deuterium gas at low pressure surrounded by a thick copper wall. Surrounding the toroidal chamber are energized windings to induce a toroidal magnetic field B.sub..phi. of about 20 kG within the chamber. Additionally, another set of energized windings induces a magnetic field B.sub.E directed along an axis which is perpendicular to the plane of the toroidal chamber and which passes through the center of curvature of the torus that defines the chamber. Initially the gas is ionized by radiofrequency signals of about 100 kHz at 20 kW. The magnetic field B.sub.E is linearly increased and adjusted at such a rate so that the external electric field is near 0.2 V/cm to minimize the appearance of runaway electrons. This field drives a plasma current that heats the plasma and rises linearly until the plasma current reaches a value of I.sub.P =100 kA and the discharge duration reaches about 100 msec. The plasma is thereby heated to about 6.times.10.sup.6.degree. K.
Various modifications and improvements on such machines were made over the years until in 1975 the Alcator tokamak of MIT achieved the values of: n=4.times.10.sup.14 atoms/cm.sup.3 ; and .tau.=18 msec.; n.tau.=0.7.times.10.sup.13 atoms-sec/cm.sup.3 ; and T=10.times.10.sup.6.degree. K.
Lately these values have changed such that n and .tau. have increased by about 50% so that n.tau. has doubled but T has decreased by about 30%.
Presently, tokamaks are rather close to the desired parameters of n.tau. and T. Some scaling laws suggest that the desired values of n.tau. will be reached with a B.sub..phi. of about 150 kG because the n.tau. product is proportional to B.sub..phi..sup.4. This is about twice the field achieved until now. Such high fields exert tremendous forces. Indeed the 150 kG field exerts a force of 6.5 tons/in.sup.2. In the Alcator tokamak the highest fields achieved are about 85 kG which exerts a force 1/3 of this value. These high fields are technologically achievable, but very expensive to generate. Another problem with present tokamaks is that the plasma pressure contained is somewhat less than 1% of the magnetic pressure. Thus 2 tons/in.sup.2 of magnetic pressure contain a plasma pressure of 20 lbs/in.sup.2. This is bad not only because of the high capital costs of the magnetic fields, but also because at these low values of .beta. (ratio of plasma pressure to magnetic pressure) the dominant loss from the plasma is synchrotron radiation, which has been neglected in obtaining the values of n, .tau. and T of the Lawson criterion mentioned above.
Inclusion of this loss due to synchrotron radiation alters the Lawson Criterion to higher values of T and furthermore the fusion power output comes uncomfortably close to the synchrotron radiation output. Thus the margin for inefficiencies is much less. Detailed calculation by many authors has shown that the synchrotron radiation problem is minimal for a .beta. of at least 0.1. This value would also minimize the capital cost of a reactor. Thus values of about 20 times the best present tokamak values are needed.
These values are partly related to the main method of heating the plasma, by driving a current of 100,000 A through it. Use of the neutral beam technology provides modest increases in .beta., perhaps factors of 2, but larger values seem unlikely.